Micro-hotplate structures can be fabricated on a semiconductor substrate and these devices are commercially manufactured. Such structures include a micro-heater embedded within a thin dielectric membrane of a dielectric region, typically comprising silicon dioxide and/or silicon nitride. The membrane of the micro-heater is produced by both wet and dry etching of the semiconductor substrate from the backside or front-side.
A reasonably comprehensive background in a paper entitled “Technological Journey Towards Reliable Microheater Development for MEMS Gas Sensors: A Review”, published in the IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 14, NO. 2, June 2014, page 589 to 599, gives a good summary of the technology. This review paper identifies issues such as materials, geometry and reliability of micro-heaters and how they are being addressed.
Most of the micro-hotplate devices reported in the literature are not fabricated in a standard microelectronics technology. The microelectronics technology is referred to in a generic form as CMOS technology as this is a well-established technology to fabricate Integrated circuits.
The CMOS term is well known in microelectronics field. In its wide meaning, it refers to the silicon technology for making integrated circuits. CMOS ensures very high accuracy of processing identical transistors (up to billions), high volume manufacturing, very low cost and high reproducibility at different levels (wafer level, wafer to wafer, and lot to lot). CMOS comes with high standards in quality and reliability.
There are many books and articles describing CMOS and there are many variants of CMOS technologies and devices that can be fabricated using CMOS technology. A very basic reference to CMOS can be found in Wikipedia (https://en.wikipedia.org/wiki/CMOS): Complementary metal-oxide-semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits, CMOS technology is also used for several analog circuits such as image sensors (CMOS sensor), data converters, and highly integrated transceivers for many types of communication. Frank Wanlass patented CMOS in 1963 (U.S. Pat. No. 3,356,858). Besides digital applications, CMOS technology is also used in analog applications. For example, there are CMOS operational amplifier ICs available in the market, Transmission gates may be used instead of signal relays. CMOS technology is also widely used for RF circuits all the way to microwave frequencies, in mixed-signal (analog and digital) applications.
There are some reports of CMOS based micro-hotplates. For example, Suehle et. al. “Tin Oxide Gas Sensor Fabricated Using CMOS Micro-hotplates and In-Situ Processing,” IEEE Electron Device Letters 1993, F. Udrea et. al. “Design and simulations of SOI CMOS micro-hotplate gas sensors,” Sensors and Actuators B 2001, M. Afridi Et. al, “A monolithic CMOS Microhotplate-Based Gas Sensor System,” IEEE Sensors Journal 2002, U.S. Pat. No. 5,464,966, M. Graf “CMOS microhotplate sensor system for operating temperature up to 500° C.” Sensors and Actuators B 2005, S. Z. Ali et, al, “Tungsten-Based SOI Microhotplates for Smart Gas Sensors” Journal of MEMS 2008, all report different examples of micro-hotplates fabricated in CMOS technology. Other reports by these same groups give information on similar devices, using polysilicon, MOSFETs, Single Crystal Silicon, and tungsten as heater materials.
There are many applications for micro-hotplates that can be found in literature. The most common use of micro-hotplate devices is for gas sensors. Reference to some of these are given in I. Simon et. al, “Micromachined metal oxide gas sensors: opportunities to improve sensor performance,” Sensors and Actuators B (2001), and S. Z. Ali Et. Al, “Tungsten-Based SOI Microhotplates for Smart Gas Sensors” Journal of MEMS 2008.
Micro-hotplates are also used as IR emitters. For example, Parameswaran et. al. “Micro-machined thermal emitter from a commercial CMOS process,” IEEE EDL 1991, and San et. al. “A silicon micromachined infra-red emitter based on SOI wafer” (Proc of SPIE 2007), describe IR emitter devices based on micro-hotplates on either suspended bridges or membranes
One of the main problems with the device fabrication is to maintain accurate tolerance on the size of the dielectric membrane during substrate etching as this can vary between 10-20 um for dry etching, and considerably more for wet etching. The size of the membrane directly affects the electro-thermal properties (such as voltage and power consumption requirements) of the micro-hotplate, as well as device reliability and yield. These problems are identified in literatures, but the method and implications of this variation in terms of the performance of devices and how to address them has never been reported. Specifically, variation in the size of membrane can affect power consumption, thermal mass and residual stress.
It is an object of the present invention to address the problems discussed above.